In recent years, interactive whiteboards for which fingers, electronic pens, and the like are used as input means have come into widespread use. Interactive whiteboards can electronically realize operability that is similar to the operability for when something is written on a blackboard with chalk. An interactive whiteboard includes a coordinate input device and a display device. In addition, the coordinate input device includes a position detection device that detects the position of an operation input by an input means, and an arithmetic unit that processes information on the position.
For the display device, a PDP (Plasma Display Panel), a LCD (Liquid Crystal Display), a projector, or the like is used. For the position detection device, tablet or touch-screen detection devices have started to come into widespread use. As a technology for realizing this, various technologies such as an electromagnetic induction method and an ultrasonic method have been put into practical use.
Besides, among position detection devices, there is also known an optical detection device that captures an image of an area around a display screen (hereinafter also referred to as an “operation input screen”) using an image sensor, and detects the position of an input object that has passed transversely across the operation input screen through signal processing on the captured image (see Patent Literature 1 and 2). The optical detection devices have been increasingly used year by year due to their advantages in high response to drawing, high tolerance to external noise such as infrared rays, sunlight, and changes in the temperature, and the like.
The optical position detection device includes image sensors, a control board for processing images acquired by the image sensors, and light sources, and such components are arranged around an operation input screen. For the optical position detection device, two or more image sensors are arranged around the operation input screen. This is in order to detect an input object that has passed transversely across the operation input screen or a shadow thereof from a plurality of visual fields, and detect the input coordinates in accordance with the principle of the triangulation.
A plurality of light sources that emit light rays such as LEDs (Light Emitting Diodes) are arranged around the image sensor. The light sources are mounted substantially in parallel with the operation input screen so that they irradiate a reflective plane with retroreflective ability that is arranged in a manner surrounding the operation input screen. Usually, for a reflective plane with retroreflective ability, a retroreflective tape is used that has minute corner cube retroreflectors and the like uniformly arranged thereon. The retroreflective tape can reflect a light ray back in the direction of a light source independently of the incident angle of the light ray. Hereinafter, a frame that has a retroreflective tape attached thereto along at least one side thereof will be referred to as a retroreflective frame.
That is, an optical position detection device adopts a structure in which a light ray is shone along an optical input screen, and a reflected ray that is reflected back by a retroreflective tape is received (captured) by image sensors. Therefore, when an input object has been brought into proximity with the operation input screen has passed transversely across the operation input screen (i.e., has blocked the light ray), a shadow appears on the captured image. The optical position detection device provides a control board with a position on the shadow detected by the plurality of image sensors, so that the coordinates of the input object that has passed transversely across the operation input screen are calculated.